1. Field of the Invention
The invention relates to a digital data packet switching module, comprising a plurality of input buses and at least one output bus, crosspoint switches connecting each individual input bus to the output bus, each crosspoint switch comprising a writing circuit that is connected to the output bus to write packets into empty packets on the output bus, and allocation means for solving access conflicts between packets intended to be transferred to the same output bus within the same packet interval.
2. Description of the Related Art
A switching module of this type is known from U.S. Pat. No. 4,821,258. The switching module set out in that patent is used in a telecommunication network for packet switching. When packets are switched in the switching module, access conflicts may arise when packets are to be transmitted over a plurality of input buses within the same packet interval.
In the switching module as claimed in said United States Patent the allocation means for solving access conflicts comprise a First-In-First-Out (FIFO) queue store per input bus followed by a code generator. This code generator provides each packet with a column code on the basis of origin and/or destination, which code corresponds with the output bus (column) to which the packet is to be transmitted. The code generator likewise produces a priority code, also on the basis of origin and/or destination. In each packet interval the priority codes of the packets that have to be transmitted to the same output bus are mutually compared per output bus and the packet having the highest priority is transmitted. Nothing can be derived from the above patent about the packets having a lower priority.
The switch function, switching packets over from the input buses to the output bus in the order they come in, is also referred to as a method according to the strict FIFO (First-In-First-Out) principle. A feature of the strict FIFO principle is that the probability of delay and loss is the same for each packet. Also if the method according to the strict FIFO principle is implemented, the probability of delay and loss of a packet is smaller with a constant packet supply than with another switching method.
It is an object of the invention to provide a switching module approximating the strict FIFO principle for transmitting the supplied packets from the input buses to the output bus in the order they come in.